As is well known, microwave ovens cook food by a different principle than conventional gas or electric ovens. With conventional ovens, the oven cavity is generally preheated to a particular temperature and the food is placed therein for a specified time period during which the heat conducts inward in the food. The relatively high temperature on the surface of the food causes browning or searing of the surface of the food. With microwave ovens, the food body is generally heated throughout by molecular agitation in the microwave field. Because the surface temperature and exposure time are considerably less than with conventional cooking, microwave ovens do not provide browning or searing of food. There has been a recognized need to enhance the appearance of some foods cooked in microwave ovens and thereby increase their palatability.
One prior art approach to the appearance problem was the introduction of combination ovens. More specifically, the food is cooked simultaneously by microwave energy and heat provided by a conventional electric element or gas convection. Combination ovens have enjoyed consumer acceptance and success in the marketplace. They provide much more rapid cooking as characterized by microwave heating and the appearance of the food is satisfactory. Combination ovens, however, are relatively expensive and there is a need to provide the consumer who has already purchased a microwave oven with a means for browning or searing food.
Even before the introduction of combination ovens, many browning dishes were available to the consumer. Generally, these dishes converted microwave energy to heat which, by conduction, presented relatively high temperatures to the surface of the food. Two materials that are used to convert microwave energy to heat are ferrites and resistive films. The ferrites may be particulate with a particular size as determined by the microwave frequency. Also, the ferrites may be dispersed throughout a medium which may be part of a microwave transparent dish or be in a form of a monolithic ceramic body which may constitute all or part of the dish. The resistive films, which have also been referred to as semiconductor or electroconductive films in the art, generally cover a portion of the outside surface of a dish. The most common resistive film is tin oxide. In operation, the dish is placed in the microwave oven to preheat and then the food is placed in the dish. This method provides some searing or browning but not to a degree to satisfy most consumers. The problem is twofold. First, as soon as the food is placed in the dish, a substantial portion of the microwave energy in the microwave field is coupled to it leaving much less to be converted to heat in the resistive film. The result is that the temperature provided by the resistive film is much less than with the no-food load condition. Second, because the food cooks much faster in a microwave oven than in a conventional one, the exposure time period to the browning heat is much less than with a conventional oven.